1. Field of the Invention
This invention relates to coatings for controlling the rate of release of drugs from implantable medical devices such as stents.
2. Description of Related Art
In the field of medical technology, there is frequently a necessity to administer drugs locally. To provide an efficacious concentration to the treatment site, systemic administration of medication can produce adverse or toxic side effect for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site.
In the treatment of vascular disorders, such as arteriosclerosis, intracoronary stents are now a standard adjunct to balloon angioplasty. Stenting eliminates vasospasm, tacks dissections to the vessel wall, and reduces negative remodeling. Stents can be made from interconnected struts that are usually between 50 and 150 microns wide. Being made of a metal (for instance, stainless steel), bare stents have to be modified so as to provide a means for local drug delivery. Accordingly, stents are being modified by forming a polymer coating, containing a drug, on the surface of the stent.
A coating used to achieve local drug delivery via stent can include a three-layer structure. The three layer structure includes a drug-polymer layer serving as a reservoir for the drug, an optional primer polymer layer for improving adhesion of the drug-polymer layer to the surface of the stent, and an optional topcoat polymer layer for reducing the rate of release of the drug. The medicine to be administered will have a sustained release profile from drug-polymer layer through the topcoat polymer layer.
To the extent that the mechanical functionality of stents has been optimized in recent years, it has been determined that continued improvements could be done by means of pharmacological therapies. For the purposes of pharmacological therapy, it is important to maintain the concentration of the drug at a therapeutically effective level for an acceptable period of time. Hence, controlling a rate of release of the drug from the stent is important, especially in such a way so as to decrease the release rate of the drug from the matrix. In view of the foregoing, coatings for reducing the rate of release a therapeutic substance from implantable devices, such as stents, are desired. The coatings should prolong the residence time of the drug in the patient, among other useful functions.